(1) Field
The present application generally relates to a method and device for separation of sulfur dioxide from a gas. In particular, the present application is directed to a method and device for separation of sulfur dioxide from a gas, which includes one or more recirculation pumps for pumping an absorption liquid to a spray tower and across an apertured plate.
(2) Description of the Related Art
Sulfur dioxide and other acid gases, e.g., sulfur trioxide, hydrochloric acid gas, and hydrogen fluoride gas form in the oxidation of sulfur-containing materials, such as coal, oil, natural gas, industrial and domestic waste, peat, etc. Sulfur dioxide and other acid gases can also form as a residual product in chemical processes, for instance metallurgical processes. Normally, it is not permitted to emit large amounts of sulfur dioxide and other acid gases into the atmosphere, which means that some kind of cleaning is necessary. One example of this is flue gas cleaning in power plants and other combustion plants. The flue gas that forms in combustion in such plants is usually cleaned, among other things, by absorption of sulfur dioxide and other acid gases into absorption liquid. The absorption liquid can, for instance, contain water and one or more of the substances lime, limestone, dolomite, sodium hydroxide solution, and similar substances, which are suitable for the absorption of sulfur dioxide. The flue gases can, for instance, be cleaned in a spray tower as disclosed for instance in EP 0 162 536, or by means of a perforated tray, as disclosed for instance in U.S. Pat. No. 5,246,471. However, these devices for cleaning gases, in particular flue gases, from sulfur dioxide and acid gases have been found to require a great deal of energy as large amounts of absorption liquid is pumped at a relatively high pressure.
U.S. Pat. No. 4,099,925, U.S. Pat. No. 5,660,616, U.S. Pat. No. 4,239,515, and WO 96/00122 describe cleaning apparatuses with low energy consumption. The flue gas is conveyed upwards through an apertured plate, on which a flowing layer of an absorption liquid is provided.
If the flue gas is not saturated with water vapor, water will evaporate from the absorption liquid and be added to the flue gas during the cleaning process. It has been found that this evaporation partially takes place when the flue gas passes through the apertured plate. One problem is that substances, such as lime, limestone, gypsum, calcium sulfite, sodium sulfate, etc, which are dissolved or suspended in the absorption liquid, tend to be evaporated and precipitated on the underside of the apertured plate and in the holes of the apertured plate. This increases the pressure drop across the apertured plate and makes the pressure drop vary over the area of the apertured plate. This results in increased energy consumption due to the increased pressure drop and in reduced absorption of sulfur dioxide due to the uneven distribution of flue gas in the layer of absorption liquid on the apertured plate. The prior-art solution to this problem is to arrange, before the cleaning apparatus with the apertured plate, a cooler in the form of a separate spray tower, for instance of the type disclosed in U.S. Pat. No. 5,753,012. In the separate spray tower, into which the flue gas is first introduced, an aqueous liquid is injected at a ratio (also called L/G) of the flow of liquid to the flow of flue gas of typically about 0.2 to 1 liter of liquid/cubic meters of flue gas and at such a high pressure that the liquid is atomized and saturates the flue gas with water vapor. After being saturated with water vapor, the flue gas can be passed through the apertured plate without the risk of solids being precipitated. A separate spray tower is however, a complicated and energy-consuming solution, which comprises pumps, pipes, tanks, control systems and a separate tower. In addition, when using such a spray tower semi-dry particles can form, which adhere to the underside of the apertured plate. It is therefore sometimes necessary to arrange a system for intermittent washing of the underside of the apertured plate.
U.S. Pat. Nos. 4,263,021, 5,281,402, and 6,923,852 are directed to systems including trays for improving gas-liquid contact. These systems include a conventional mechanism for moving an absorption liquid to the top of the tray, e.g., by pumping the absorption liquid through a conduit to nozzles positioned above a top surface of the trays. Accordingly, the flow rate of gas flowing in such systems is limited by the geometry of the ductwork and vessel, the size of the pump, and the number of nozzles. In addition, these systems operate under dual flow conditions where the absorption liquid drips through the holes in the tray rather than by gravity through a conduit.
Referring now to FIG. 1, U.S. Pat. No. 7,094,382, which is hereby incorporated by reference as if disclosed herein in its entirety, describes a device 20 for separating sulfur dioxide from a gas 21, which includes a spray tower 22 having an inlet duct 24 for the gas, an outlet duct 26 for the gas, from which sulfur dioxide has been separated, and an apertured plate 28 arranged between the inlet and the outlet. The gas passes through apertures 30 in apertured plate 28 from below. A flowing layer of absorption liquid 32 is supported by an upper side 34 of apertured plate 28. An inlet duct 36 connects a container 38 for absorption liquid to upper side 34 of apertured plate 28. A pump 40 is utilized to introduce compressed air 42 into the absorption liquid thereby serving as an airlift, which conveys the absorption liquid from container 38, through inlet duct 36, to upper side 34 of apertured plate 28, and along the apertured plate. The absorption liquid flows back to container 38 through apertures 30 and through an opening 44 in an outlet box 46 at end of apertured plate 28 opposite inlet duct 36. The absorption liquid flowing from opening 44 creates a quench waterfall 48, which contacts gas 21 flowing into device 20 from inlet duct 24. In U.S. Pat. No. 7,094,382, the flow rate of gas 21 is limited by the geometry of inlet duct 36, the geometry within a portion of spray tower 22 that contains flowing layer of absorption liquid 32, which is supported by upper side 34 of apertured plate 28, and the size of the pump(s) 40.